Pliable sheets of thermally sealable materials are commonly used in many industrial, environmental and civil construction applications. For instance, a single-ply thermo-plastic sheet is utilized as a roofing membrane to prevent leakage of water through a roof. Thermo-plastic sheets are also used as pond and landfill liners to contain hazardous waste and to prevent such waste from seeping into the ground. Typically, in such applications, large sheets of thermo-plastic materials are placed on the roof or in a containment pond with their edges overlapping. The overlapping edges are seam-welded or tack-welded to one another along or within the overlap. This sealing or tack-welding process is done manually by an artisan who separates the overlapping edges along the overlap and then heats the separated edges by directing hot air between the separated edges with a blowing gun or hot air device. When heated sufficiently, the surfaces of the separated edges liquify. The edges are then pressed together to provide a seam or tack-weld between the overlapping edges which joins the sheets together when it cools. As can be appreciated, such hot air techniques are very time-consuming and, therefore, expensive.